High viscosity finisher

ABSTRACT

A self-wiping multiple screw element mixer of conical configuration useful as a polymerizer/finisher for the preparation of high viscosity polymers such as polyamides, polyesters, etc.

United States Patent 1 Pinney 1 Feb. 20, 1973 [54] HIGH VISCOSITYFINISIIER [56] References Cited [75] Inventor: Baden McDowall Pinney,Kingston, UNITED STATES PATENTS Ontario, Canada 617,735 1/1899 Godfrey..259/6 X Asslgneer Du Pom of Canada, Limited, 3,226,097 12/1965 VaydaetaL. ....259/6 x treal, Canada 3,343,922 9/1967 Zimmer et al. ...2S9/6X [22] Filed: June 1971 Primary Examiner-Robert W. Jenkins [21] Appl.No.: 149,580 Assistant Examiner-Philip R. Coe

Attorney-Herbert W. Larson [30] Foreign Application Priority Data [57]ABSTRACT June 8, Canada A se ip g sc w e e ent ixer of conicalconfiguration useful as a polymerizer/finisher for the [52] US. Cl..259/6 preparation of high viscosity polymers Such as [5 I 1 Int. Cl.mides, p y etc [58] Field of Search.....259/6, 104, DIG. 30; 15/93 R 6Claims, 5 Drawing Figures PATENTED FEBZ 01973 FIG. M30

INVENTOR BADEN M. PINNEY ATTORNEY PATENT [[1 FEBZO ms SHEET 2 0F 2INVENTOR BADEN M. PINNEY ATTORNEY HIGH VISCOSITY FINISHER SUMMARY OF THEINVENTION This invention relates broadly to a mixing apparatus and moreparticularly a self-wiping multiple screw element mixer of conicalconfiguration useful as a separator/finisher for producing polymers suchas polyamides, polyesters, etc.

The term mixing used herein includes finishing high viscosity syntheticpolymers, mixing two or more viscous liquids and blending solids andliquids together.

BACKGROUND OF THE INVENTION In the preparation of synthetic polyamidesor polyesters, bifunctional amide-forming or ester-forming monomers arecontinuously heated, often under conditions of reduced pressure, topromote the condensation reaction and to assist in the removal ofvolatile by-products until the desired degree of polymerization isattained, as indicated, for example, by viscosity measurement ormolecular weight.

The term reduced pressure used herein refers to the condition thatpromotes the vaporization of reaction by-products. This condition may beobtained by creating a partial vacuum or by the use of an inertatmosphere.

Polyamides and polyesters may be prepared by either a batch process or acontinuous process. Various of either methods for preparing thesepolymers are known in the art. For example, in the case of a polyamide aconcentrated aqueous solution of the amide-forming reactants, such as asalt of a diamine and a dicarboxylic acid, can be supplied to a reactorwherein the temperature and pressure conditions are such that a majorportion of the salt is converted to polymer. In one type of continuouspolymerization process, the reaction mass is then fed to a flashervessel where heat is added to maintain the temperature, but the pressureconditions are reduced thus permitting the separation of water from thereaction mass as steam. Finally, the polymer is fed to aseparator/finisher, hereinafter referred to as a finisher, wherein thedesired degree of polymerization is obtained.

In a typical case for preparing a polyester, an esterforming monomer,such as bis(hydroxyalkyl) terephthalate, which has been prepared by theester interchange of a lower dialkyl ester with alkylene glycol, issubjected to certain conditions of pressure and temperature, in thepresence of a catalyst, to promote polymerization. Provision is made toremove the volatile by-products, such as methanol, and any excessalkylene glycol that may be present. In a continuous polymerizationprocess, the reaction mass is normally passed through at least twovessels before the desired degree of polymerization is obtained. Eachvessel usually has a higher temperature and a lower pressure than thevessel preceding it. The final vessel is often referred to as a finisherand the polymer leaving the finisher is in a condition where it can beusefully extruded into a film or spun into filaments.

The partial pressure of the volatile by-products is reduced during thefinishing step either by the use of an inert atmosphere or by finishingunder a partial vacuum. However, there is usually a long retention timerequired in the finisher for the polymer to reach the predeterminedmolecular weight. This long retention time often leads to deposits ofby-products such as degraded polymer on unwiped surfaces in thefinisher. In the case of polyamides, these degraded polymers aresometimes referred to as gel.

In conventional finishers of the intermeshing screw design, used in thepreparation of polyamides, the gel problem is overcome as the screws areself-wiping. However, most of these conventional finishers do notprovide a long retention time for the polymer, hence they must beredesigned to have a greater volumetric capacity, and this redesign canresult in an impractical or expensive finisher. Furthermore, where thepolymer entering the finisher has a high moisture content, foaming orflooding of the venting section of the screw may occur because of thelimited vapor disengagement space provided in conventional screwfinishers. For large diameter, fully wiped, deep channel screwfinishers, the screw volume to free volume ratio is high thus limitingthe maximum use of the vessel space. Another disadvantage of this typeof fully wiped deep channel screw finisher may arise due to poor heatdissipation. This is caused by high viscous shear occurring oppositelimited cooling capacity.

In conventional finishers comprising large vessels, often conical inshape, having ribbon type or paddle type agitators, the problem ofhaving very long screws is overcome, as well as the problem of foamingor flooding the venting section, as a large vessel can have ample vapordisengagement space. However, as the agitators are not self-wiping, geldeposits build up on the unwiped surfaces, and the vessel has to becleaned out at regular intervals.

DETAILED DESCRIPTION OF THE INVENTION It is therefore an object of thepresent invention to provide a mixing apparatus having self-wipingfeatures.

It is a further object to provide a mixing apparatus for preparingpolymers which eliminates the disadvantages of the conventionalfinishers while retaining and combining the main disadvantages of alltypes.

An additional object of the present invention is to provide a mixingapparatus having a mixing zone and a pumping zone.

With these and other objects in view, there is pro vided an apparatussuitable for mixing comprising: a vessel having an interior surfacethroughout its length in the shape of at least two intersecting conicalfrustums with axes parallel and substantially vertical, the base of thefrustums being displaced upwards with respect to the apexes, at leasttwo interengaging helical screw elements rotatably mounted within thevessel which when co-rotated conform to the interior surface of thevessel such that the screw elements effect a complete cleaning of theinterior surface and wherein the screw elements interengageuninterruptedly along their length such that each element effects acomplete cleaning of the adjacent elements.

THE DRAWINGS In the drawings which illustrate embodiments of theinvention:

FIG. 1 is a vertical sectional view of one embodiment of a mixingapparatus of this invention.

FIG. 2 shows a cross-sectional view of the interengaging screw elementsat 22 in FIG. 1.

FIG. 3 shows a cross sectional view of the interengaging screw elementsat 3-3 in FIG. 1.

FIG. 4 shows a cross-sectional view of the interengaging screw elementsat 4-4 in FIG. 1.

FIG. 5 shows an end view of one embodiment of a drive mechanism for theinterengaging screw elements of the present invention.

The mixing apparatus illustrated in FIG. 1 includes a vessel havinginterior surface 11 which takes the shape of two intersecting frustums.of cones with parallel axes. The axes are generally substantiallyvertical and the base of each of the cones is displaced upwards withrespect to the cone apexes. Under some conditions it may be desirable tohave the axes of the vessel tilted from the vertical. Surrounding theinterior surface 11 is a heating jacket 13. The heating jacket 13 shownis for a vapor or liquid heating medium such as Dowtherm registeredtrademark of Dow Chemical. Under some conditions the jacket 13 may coolthe vessel to stop the temperature rising above a preset level.Alternatively, an electrical heating jacket may be substituted, thevessel wall 1 1 may be finned externally for heating or cooling by air,or in certain cases neither jacketing nor finning is required.

Inside the vessel are two co-rotating, interengaging screw elements 14connected to shafts 15 which pass through seals 16 in the base 17 of thevessel. The screw elements 14 are self-wiping for their full length. Thescrew elements 14 also wipe the entire interior surface 11 of thevessel, including the top and bottom plate of the conical frustums. Thegeometries of the screw elements from the bottom of the screws, i.e., atsection 22 up to section 3-3, is such that pressure generatingcharacteristics are obtained. These pressure generating characteristicsneed only occur from the bottom of the screws up to approximatelyhalfway between section 2-2 and section 3-3. The geometric developmentof these portions of the twin screws incorporating self-wiping featuresis well known in the art, although in the past it has been more frequentto employ these features in parallel rather than tapered screws. Thereare many types of intermeshing screw configurations, some having asingle start, some having multiple starts, which are all suitable forthis application. This section of the vessel between section 2-2 andsection 3-3 is hereinafter referred to as the pressure generating zone.

At section 3-3 the core radii are equal to their center to centerdistance and this marks an important transition in the character of thescrew elements. In the pressure generating zone below section 3-3 manytypes of self-wipin g twin screw configurations are possible (asindicated in the previous paragraph), but above section 3-3, there areonly a limited number of selfwiping screw configurations. One suchconfiguration is shown in FIG. 4.

Above section 3-3 the conical vessel increases in diameter, but thecross-sectional area of the screw elements remains constant. The vesselscrew relationship begins to change in character in that a hollow center18 shown in FIG. 4 starts to appear by virtue of the increasing diameterof the conical shape of the vessel. This so-called hollow center 18 isnot traversed by any part of the screw elements. The maximum size ofhollow center 18 occurs at the top of the screw elements.

This portion of the vessel above section 3-3 to the top of the screwelements is referred to as a mixing zone. In the case of a finisher,mixing and circulation of the polymer occurs in the melt pool at thelower part of this zone, and thin film generation and vapor diffusionoccurs at the upper part of the zone. As shown in FIG. 1, the liquidlevel 19 divides the mixing zone into an' upper part and a lower part.The level 19 may be raised if more mixing and circulation is required orlowered if it is desirable to have the greater part of the vessel actingas a thin film generator and vapor disengagement section.

The screw elements may be cast integrally with the shafts 15 instainless steel or other suitable material or may be separatecomponents. A helix angle is selected from a wide range to provide goodwiping of screws and wall. Other considerations in selecting the helixangle include sufficient surface area generation for mass transferpurposes and adequate mechanical strength in the screws.

In one embodiment, a part of the screw length has a helix angle(infinite pitch). This produces blades having straight elements whichare easily manufactured and retain the self-wiping features.

In the embodiment illustrated, FIG. 4 shows the cross section of twoscrew elements 14A and 14B of equal size. The cross section of eachelement is shown bounded by three equal arcs. Arcs 20A and 20B face theinterior surface 11 of the vessel, and the remaining arcs 21A, 21B, 22Aand 22B join at land edges 23A and 23B pointing towards the center ofthe vessel. All the arcs have a radius a which has its center point atthe land edge opposite each arc. This radius is approximately equal tothe center distance between the two cones. The cross section of theelements remains constant from section 3-3 to section 4-4.

As may be seen in FIG. 4, upon rotation of the two screw elements 14Aand 14B in the same direction, one land edge 23A on the screw element14A wipes the outer arc 20B of the screw element 148. The elementsrotate in the direction indicated by the arrow, and the land edge 23Amoves across the arc 20B in a direction opposite to the rotation of theelements until land edge 23A reaches land edge 24B on element 148. Atthis time the relative position of the two elements changes, and theland edge 24B commences to wipe the arc 21A of the element 14A. In onecomplete revolution of the screw elements 14A and 14B, the three arcs onboth elements are wiped together with the interior surface 11 of thevessel.

One type of drive for the two screw elements 14 is shown in FIG. 7. Twogear wheels 25 having the same number of teeth are joined to shafts 5and a drive gear 26 drives both gear wheels 25 in the same direction, atthe same speed and without any slippage occurring. The drive gear 26 isconnected to a shaft 26A which in turn is driven at a constant speed bya motor through a standard gear reduction. It has been found that incertain cases with careful selection of the helix angle of the screwelements the gear wheels may be omitted and if one screw element isdriven the second screw element is rotated through its engagement withthe driven screw. The speed of rotation of the screw elements 14. isselected on the basis of adequate shear rates in the operatingclearances to prevent surface deposits and in conjunction with mixingefficiency, thermal effects, and pressure requirements at the vesseldischarge. The speed must be sufficient to move material from the vesselinterior surfaces and screw element surfaces under the influence of dragflow and at the same time force material out of the discharge at thedesired flow rate. A preferred speed is in the order of 20 rpm.

in many cases, gravity forces are sufficient to promote the flow ofmaterial towards the discharge of the vessel thus permitting the use ofinfinite pitch screw elements in at least ;art of the vessel wherereduced surface area can be tolerated.

1n the embodiment shown, a top plate 27 encloses the vessel 10, and hasa heating jacket 28 for a liquid heating medium. The liquid materialenters the vessel through inlet pipe 29. The vapor by-products releasedin the vessel pass out at a vent 30 in the top plate 27. This vent 30 isshown in line with and the same size as the hollow center 18 at the topand between the screw elements 14. The final product discharges at theend of the pressure generating section atthe bottom of the vessel 10through a discharge pipe 31.

In another embodiment, the vent 30 is fitted with a sleeve 34 extendingto within close proximity of the screw elements. The sleeve isremovable, and may be replaced when vent deposits build up to anundesirable level.

The top plate is not an essential feature of the vessel. There may beanother vessel extending above the first vessel. Furthermore, the screwelements may terminate before they reach the top of the vessel. Theymay, for instance, terminate at the liquid level 19.

In the embodiment shown, the screw elements 14 terminate in flat ends 32designed to wipe the inner surface 33 of the top plate 27. Thus depositscannot form on the inner surface 33 or the flat ends 32. Clearances inthe mixing section between the two screw elements 14, the interiorsurface 11 and the top surface 33 should be sufficient to preventdeposits forming on any of the surfaces but in the case of a finisherallow a thin film of polymer to form thus facilitating the diffusion ofthe volatile by-products. Clearances in the order of 1/32 to H4 inch aresatisfactory for a finisher. In the pressure generating section tightclearances for adequate pressurization are required. Clearances in theorder of 0.005 to l/ 16 inch are satisfactory for a finisher, but may bemore or less for other types of mixers as required.

In operation, materials to be mixed are fed into the vessel 10 throughthe inlet pipe 29. The flow of the ingredients is controlled so that thelevel 19 of the pool remains constant. The ingredients are picked up bythe screw elements 14 and deposited on the interior surface 11 of thevessel. The ingredients are then forced down into the pool by the wipingaction of the screw elements 14 and gravity leaving a thin film ofliquid on the interior surface 11 of the vessel 10 and the screwelements. This thin film is constantly being replenished, and exposed tothe vessel atmosphere, so that volatile by-products are diffused thusaiding in the mixing and processing of the ingredients.

The material in the pool is forced down the interior surface 11 by therotating motion of the screw elements 14. Excess material, which doesnot enter the pressure generating Zone, passes up through the hollowcenter 18 between the screw elements 14, and recirculatcs again down theexterior surface. Vapors given up during the recirculation pass upthrough the hollow center 18 and vent through the central vent 30.

The material entering the pressure generating zone is pressurized andpumped out through the discharge pipe 31 at the bottom of the vessel 10.

For ingredients having large quantities of moisture or gases entrainedtherein, a shaped piece may extend into the hollow center 18 between thescrew elements 14. This shaped piece would be wiped-by the inside landedges 23A and 23B of the rotating screw elements 14 and could be in theform of a baffle.

The location of the shaft seals 16, at the high pressure end of thepressure generating zone, overcomes the problem of air leakage which iscommon in conventional equipment when operating under reduced pressureconditions.

In certain conditions, some sections of the apparatus may tend to loseor alternatively generate an undesirable amount of heat. It has beenfound that by circulating a heat-exchange medium through jacketed orfinned walls of the vessel or hollow screw elements, the product in thevessel may be maintained at the desired temperature.

Modifications of the illustrated embodiment may be carried out by thoseskilled in the art. For instance, the feed may be at the bottom with anoverflow or auxiliary scavenging screw at the top for process discharge.In the case of a finisher used in batch operations, the pressuregenerating zone may be relieved by the addition of slots along the screwor wall or vessel to reduce the pressure generating capacity.Alternatively, blades having infinite pitch may be used. Kneading typesections, reverses, etc. may be used to achieve special blending orother characteristics. The mixing zone of the vessel is particularlysuitable for introducing special additives such as copolymers ordisperse materials thereby eliminating a further processing step such aspassage through a separate mixer. Cones having different diameters maybe used with suitable modification of the screw element cross sections.

The lower section of the vessel generally has a conical configuration,and the upper section while shown as a continuing conical configurationin this embodiment may be cylindrical or have a second conicalconfiguration which intermeshes back with the lower conicalconfiguration.

In the latter configuration the screws or blades may be driven fromabove. Shear rates which greatly affect mixing performance aredetermined by the clearances between the screws, and between the screwsand the walls of the vessel. Another factor involved is the relativesurface speeds between the moving surfaces. Specific mixing ordispersing effects are obtained by varying the rotational speed of thescrews or by raising or lowering the screw elements to adjust theclearances between the screws and the walls of the vessel. ln practice,it is found that making a device with screws having variable clearancesis expensive and only necessary in exceptional cases.

The screw elements shown in the present embodiment have equaldimensions, however, they may vary in size and fit in frusto-conicalsections having different diameters. The cross section of the screwelements in the mixing zone is shown bounded by three equal arcs withone of the arcs adjacent to the interior surface of the vessel. Thisconfiguration may be reoriented so one of the land edges between thearcs is adjacent to the interior surface of the vessel, thus having anedge wiping the interior surface rather than an arc. Furthermore, thegeometry of the screw elements may be altered so the cross sections ofthe elements have a circular configuration.

Process materials from the output of the pressure generating zone may berecycled to the input in batch processes or partially recycled incontinuous processes. In the case of a finisher used in the preparationof polyamides, the recycle step facilitates the removal of volatileby-products and increases the viscosity of the polymer by furtherexposure in the finisher. Recycling may be a separate system from theregular input and output shown in the present embodiment.

lclaim:

1. An apparatus for finishing high viscosity synthetic polymerscomprising:

an enclosed vessel having an interior surface throughout its length inthe shape of two intersecting conical frustums with axes parallel andsubstantially vertical, the base of the frustums being displaced upwardswith respect to the apexes, an entrance and a vent in the upper portionof the vessel and a discharge in the lower portion of the vessel;

two interengaging helical screw elements rotatably supported on shaftspassing through seals in the base of the vessel, the screw elements whencorotated conform to the interior surface of the vessel such that thescrew elements effect a complete cleaning of the interior surface andwherein the screw elements interengage uninterruptedly along theirlength such that each element effects a complete cleaning of theadjacent element, the bottom portion of the screw elements forming apressure generating zone, and the top portion of the screw elementsforming a mixing zone having a hollow center described by theco-rotating screw elements.

2. The apparatus according to claim 1 in which the vent in the upperportion of the vessel is coincident with the hollow center between theco-rotating screw elements.

3. The apparatus according to claim 1 in which the vent in the upperportion of the vessel is in the form of a removable sleeve.

4. The apparatus according to claim 1 in which the vent in the upperportion of the vessel is coincident with the hollow center between theco-rotating screw elements and is in the form of a removable sleeve.

5. A mixing apparatus comprising a vessel having an interior surfacethroughout its length in the shape of at least two intersecting conicalfrustums with axes parallel and substantially vertical, the base of thefrustums being displaced upwards with respect of the apexes, at leasttwo inter-engaging helical screw elements rotatably mounted within thevessel which when corotated conform to the interior surface of thevessel such that the screw elements effect a complete cleaning of theinterior surface and wherein the screw elements inter-engageuninterruptedly along their length such that each element effect acomplete cleaning of the adjacent element, and wherein the screwelements when co-rotated describe a hollow center in the mixing zone ofthe vessel.

6. A mixing apparatus comprising a vessel having an interior surfacethroughout its length in the shape of at least two intersecting conicalfrustums with axes parallel and substantially vertical, the base of thefrustums being displaced upwards with respect of the apexes, at leasttwo inter-engaging helical screw elements rotatably mounted within thevessel which when c0- rotated conform to the interior surface of thevessel such that the screw elements effect a complete cleaning of theinterior surface and wherein the screw elements inter-engageuninterruptedly along their length such that each element effects acomplete cleaning of the adjacent element, and wherein the vessel has atop plate having a flat inner surface and the screw elements have asubstantially flat upper surface which when corotated conform to the topinner surface of the vessel such that the flat upper surface of thescrew elements effect a complete cleaning of the top inner surface ofthe vessel.

1. An apparatus for finishing high viscosity synthetic polymerscomprising: an enclosed vessel having an interior surface throughout itslength in the shape of two intersecting conical frustums with axesparallel and substantially vertical, the base of the frustums beingdisplaced upwards with respect to the apexes, an entrance and a vent inthe upper portion of the vessel and a discharge in the lower portion ofthe vessel; two interengaging helical screw elements rotatably supportedon shafts passing through seals in the base of the vessel, the screwelements when co-rotated conform to the interior surface of the vesselsuch that the screw elements effect a complete cleaning of the interiorsurface and wherein the screw elements interengage uninterruptedly alongtheir length such that each element effects a complete cleaning of theadjacent element, the bottom portion of the screw elements forming apressure generating zone, and the top portion of the screw elementsforming a mixing zone having a hollow center described by theco-rotating screw elements.
 2. The apparatus according to claim 1 inwhich the vent in the upper portion of the vessel is coincident with thehollow center between the co-rotating screw elements.
 3. The apparatusaccording to claim 1 in which the vent in the upper portion of thevessel is in the form of a removable sleeve.
 4. The apparatus accordingto claim 1 in which the vent in the upper portion of the vessel iscoincident with the hollow center between the co-rotating screw elementsand is in the form of a removable sleeve.
 5. A mixing apparatuscomprising a vessel having an interior surface throughout its length inthe shape of at least two intersecting conical frustums with axesparallel and substantially vertical, the base of the frustums beingdisplaced upwards with respect of the apexes, at least twointer-engaging helical screw elements rotatably mounted within thevessel which when co-rotated conform to the interior surface of thevessel such that the screw elements effect a complete cleaning of theinterior surface and wherein the screw elements inter-engageuninterruptedly along their length such that each element effect acomplete cleaning of the adjacent element, and wherein the screwelements when co-rotated describe a hollow center in the mixing zone ofthe vessel.